1. Field of the Invention
The present invention relates to a magnetic refrigerator using a magnetic material having a magnetocaloric effect.
2. Description of the Related Art
The present refrigeration technique in a room temperature range related closely to human daily life, such as a refrigerator, a freezer and an air conditioner utilizes a vapor compression cycle. However, the refrigeration technique based on the vapor compression cycle has a serious problem of environmental disruption caused by emission of chlorofluorocarbons. Chlorofluorocarbon alternatives seem to have a bad influence on the environment. Under the circumstances, efforts have been taken to protect the environment by using a natural refrigerant (such as CO2 and ammonia) and isobutane with a low risk to the environment. Practical use of safe, clean and efficient refrigeration technique free from environmental disruption due to discharge of working gas has been demanded.
Recently, magnetic refrigeration has been expected as one of eco-friendly and efficient refrigeration technique, and magnetic refrigeration technique in a room temperature range has been actively researched and developed. The magnetic refrigeration technique is principally based on a magnetocaloric effect in iron (Fe) found by Warburg in 1881. The magnetocaloric effect means the phenomenon that varying an external magnetic field applied to a magnetic material in an adiabatic state causes a temperature change of the magnetic material.
Magnetic refrigeration utilizes the magnetocaloric effect to produce a low temperature. In a magnetic material, entropy is changed by the difference in the degree of free electron magnetic spin system between a magnetic-field applied state and a zero magnetic state. Accompanying with such an entropy change, entropy is transferred between the electron magnetic spin system and a lattice system. Magnetic refrigeration uses a magnetic material having a large electron magnetic spin and transfers entropy between an electron magnetic spin and a lattice system by utilizing a high entropy change between a magnetic-field applied state and a zero magnetic state, thereby producing a low temperature.
In the former half of 1900s, magnetic refrigeration systems were developed, in which a paramagnetic salt such as Cd2(SO4)3.8H2O or a paramagnetic compound represented by Cd3Ga5O12 (gardolinium gallium garnet, GGG) was used as a magnetic material having a magnetocaloric effect. The refrigeration system achieving magnetic refrigeration with the paramagnetic material is mostly applied in a very low temperature range lower than 20K, in which a magnetic field of approximately 10 tesla obtainable using a superconducting magnet is employed.
On the other hand, magnetic refrigeration utilizing magnetic phase transition between a paramagnetic state and a ferromagnetic state of a ferromagnetic material has been actively studied since 1970s to realize magnetic refrigeration in a higher temperature range. As a result of these studies, a number of magnetic materials having a large electron magnetic spin per unit area has been proposed, which includes rare earth elementary substances of lanthanum group such as Pr, Nd, Dy, Er, Tm and Gd, alloy materials such as Gd—Y and Gd—Dy containing two or more types of rare earth elements, and rare earth intermetallic compounds such as RAI2 (wherein R indicates a rare earth element, the same hereinafter), RNi2 and GdPd.
In 1974, Brown (USA) first realized magnetic refrigeration in a room temperature range using a ferromagnetic material Gd with a ferromagnetic phase transition temperature (Tc) of approximately 294K. The Brown's experiment achieved a continuous operation of refrigeration cycle, but did not reach a steady state. In 1982, Barclay (USA) thought up to use positively lattice entropy that was considered an impediment to the magnetic refrigeration in a room temperature range, and proposed a refrigeration system in which a magnetic material bears, in addition to the magnetocaloric effect for the magnetic refrigeration operation, a heat storage effect to store cold energy generated by magnetic refrigeration operation (see U.S. Pat. No. 4,332,135). This magnetic refrigeration system is called an AMR (active magnetic refrigeration) system. These refrigeration systems are operated under a strong magnetic field using a superconducting magnet.
In 1997, Zimm, Gschneidner and Pecharsky (USA) produced experimentally a magnetic refrigerator of the AMR system using a cylinder filled with fine spherical Gd grains, and succeeded in a continuous steady operation of a magnetic refrigeration cycle in a room temperature range. Specifically, they succeeded in refrigeration at about 30° C., by changing a magnetic field from 0 tesla to 5 tesla using a superconducting magnet in a room temperature range, and reported that when a refrigeration temperature difference (ΔT) was 13° C., very high refrigeration efficiency (coefficient of performance COP=15, excluding input power to the magnetic field generator) was obtained. For reference, a refrigeration efficiency of a refrigerator in a vapor compression cycle using a conventional chlorofluorocarbon is 1 to 3.
In 2000, Bohigas (Spain) reported a refrigeration system using a permanent magnet. This refrigeration system has a structure in which a wheel having a magnetic material in a shape of a ribbon placed around the wheel is inserted into a clearance between permanent magnets fixed opposite to each other and the wheel is rotated by a motor. The system uses Gd as the magnetic material, and proves refrigeration of 1.5K in a room temperature environment under the conditions of a magnetic field of 0.3 T, olive oil refrigerant, and rotation speed of 4-50 rpm. However, this magnetic refrigeration system has drawbacks of a complex structure incorporating the rotating member in a refrigerant circulation mechanism and insufficient cooling capacity.